The discovery of high temperature oxide superconductors enabled the operation of superconducting devices at liquid nitrogen temperatures, about 77K, instead of liquid helium temperatures, about 4.2K. Among the many applications for this new class of materials are power transmission lines, electric motors and superconducting magnets. Many of these applications will require the development of durable, flexible wires or tapes of the oxide superconductor.
Due to the brittleness of the oxide superconductor, oxide superconducting articles typically are formed as composites of the oxide superconductor and a ductile, inert (or noble) metal, such as silver. Silver imparts desirable mechanical properties, such as reduced brittleness, to the otherwise brittle oxide ceramic. Further, the silver matrix provides a good means of thermal dissipation and may function as an electrical shunt in the event of an electrical short. While the ductile metal provides flexibility to the composite, it is soft and prone to distortion under use conditions. An increase in strength or mechanical toughness of an oxide superconducting composite while retaining the flexibility of a silver-oxide composite is desirable.
Oxide dispersion hardening is a phenomena which imparts increased strength to a matrix material by precipitating metal oxide domains from the matrix material. Oxide dispersion hardening exists when small oxide particles (typically less than about 500 microns) are dispersed throughout the metal matrix. The hardness of the oxide domains impart strength and toughness to the matrix. These oxide particles, however, disadvantageously form sites where dislocations may be pinned in concentric Orowan Loops surrounding the oxide particle. Such pinned dislocations may be the source of material failure in the matrix metal.
Oxide dispersion hardening has been used in the field of oxide superconductivity to improve the hardness and mechanical strength of the silver-oxide superconductor composite. However, conventionally prepared oxide dispersed silver (ODS) hardened alloys often crack or fracture during conventional wire-making operations, such as rolling, drawing or pressing. This cracking may be due to reduced ductility in the precipitation hardened composite, presumably due to the formation of pinning sites during precipitation of the metal oxide domains in the silver matrix.
In a typical oxide powder in tube method (OPIT), an oxide superconductor powder is introduced into an ODS sheath. Multiple wire forming operations are performed in order to obtain a wire or tape having the dimensions, density and texture required of the oxide superconductor composite. See, for example, Sandhage et al. JOM, 21 (March, 1991). These processing operations are hindered by the brittle nature of the ODS sheath and often result in undesirable cracking of the composite. In order to avoid cracking, additional processing steps involving smaller deformation stresses are used. This adds to processing time and costs.
Alternatively, U.S. Pat. No. 5,384,307 to Lay describes preparation of a superconducting article in which an oxide superconductor core and a silver alloy sheath (including a solute metal) are reacted in an oxidizing atmosphere consisting of three to fourteen volume percent oxygen. The solute metal of the silver alloy sheath is converted to a metal oxide under these reaction conditions. In addition to the drawbacks previously mentioned which are associated with increased sheath brittleness, ODS hardening processes such as disclosed by Lay may degrade the oxide superconductor's electrical properties. In particular, the process may alter the oxygen stoichiometry of the oxide superconductor because the solute metal, typically magnesium or aluminum, is so reactive that it will react with the core oxide superconductor such that the oxide superconductor is reduced at the expense of the solute metal of the silver alloy. Thus, the oxygen of the oxide superconductor may be depleted and oxygen stoichiometry is altered.
The ODS hardening reactions of the prior art have deleterious effects on the composition, process ability and electrical performance of the oxide superconductor article. There remains a need for an oxide superconductor composite with increased strength and hardness without the limitations of the prior art.
It is an object of the invention to provide an oxide superconductor article of improved durability and ductility having a sheath with oxide particles disposed therein.
It is an object of the invention to provide an oxide superconductor composite having improved handling capability due to increased hardness and reduced brittleness.
It is yet another object of the invention to prepare an ODS hardened oxide superconductor composite without deleterious effect to the mechanical and electrical properties of the oxide superconductor.
It is a further object of the present invention to obtain an oxide superconductor composite which possesses improved electrical performance, such as critical current density.
It is yet another objective to obtain ODS BSCCO tapes with improved texture.